Enthalpy and Heat Calculation

[aisers]

Hello!!

I was wondering if i might be able to get some help/advice on this query I have. I have an excel spreadsheet which calculates the enthalpy change and heat required when chnaging the pressure of a gas sample in a gas pipeline. However I am unable to fully understand where some of the figures are being taken from:

This is the formula that appears to be being used to calculate the enthalpy of the gas coming in:

H= [(c-E1)*Tin]+[D(Tin^2)]+[(A-E2)*P1]-[B*P1/(Tin^2)]

Where:
Tin = Temp. of the gas entering
Pin= Pressure of the gas entering
A= -0.00792078
B= 950.85
C= 0.0314566
D= 0.000009138
E1= 0.000358
E2= 0.001391

There is also a figure called ERF (which I presume is some type of correction factor) which is added to the enthalpy, this is calculated by:

ERF=E3*(P1^2)+(E4*P1*Tin)+(E5*Tin*P1^2)+E6

Where:
E3= 0.0003572
E4= 0.000008397
E5= 0.00000119
E6= 0.0845

the Change in enthalpy is found by taking the total enthalpy (including correction factor) of the gas leaving the pressure changer from that of the gas entering the pressure changer.

I cannot understand what the values A,B,C,D, E1 ...... etc. are. They are simply numbers in the spreadsheet I have been given.

Any help or advice on where they might have come from would be much appreciated - Sorry for the long post!

Aisers

 

[Latexman]

Most likely, someone has statistically fit a theoretical or emperical equation to enthalpy data for the gas you are working with. Since you did not identify the gas, it's almost impossible for us to help you. I'd recommend asking the person that gave you or made the spreadsheet.

Good luck,
Latexman

 

[ione]

Why don’t ask this to the person who passed on to you the spreadsheet (He/she should know what it has been reported in). It’s practically impossible to give an answer.

 

[aisers]

Sorry its for a natural gas pipeline, The person who gave me the spreadsheet cannot identify where they have come from either as this is a very old calculation.

I have found one other person who seems to be using these forumlae on another forum - His question was about the actual coding of the program and not the theory. I have messaged him but have gotten no reply.

http://www.mrexcel.com/forum/showthread.php?p=2797975

 

[ione]

Latexman,

you've definitely been faster than me

 

[Latexman]

Maybe zdas04 knows where these equations originate. He's been in oil and gas a few years.

Good luck,
Latexman

 

[pmover]

the equation does not resemble any equation of state for enthalpy that i've seen or know of (BWR, PR, RK, etc.).

strange that the (c-E1) value is not a single constant (must be pre-determined). typically, the constants for enthalpy equations are different for each hydrocarbon gas (methane, ethane, etc.) and then further calculations are accomplished to obtain the enthalpy value at the respective P & T. i wonder if the equation provided is for a single hydrocarbon gas.

the suggestion by Latexman is recommended.

good luck!
-pmover

 

[aisers]

Hey Guys!

Thanks for your feedback - I really appreciate you having a look at it, I know there is next to nothing to go on.

I believe this equation/calculation has been used in the gas industry for a good number of years by companies such as british gas etc.

It just seems weird that no one is able to place it or decipher where it comes from.

Thanks for your help,

Aisers

 

[adhy83]

that's merely numeric equation...but for current situation,you may use Hysys to check the formula accuracy

 

[xeng]

adhy83, do not forget this is a forum for engineers which (possibly) wish to understand the methods they use in order to explain the results.
The suggestion to purchase a simulator to verify a formula could appear, at least, exaggerate.

 

[davefitz]

Interesting post. Seems to be another example of computer programs that were originally programmed in the 1960's converted over to PC programs after the original programmer retired. Massive documentation problem industry wide. Ultimately new piping systems are being desinged with some part of the calcualtion based on "magic"- undocumented bases.